Prior techniques of forming edges on contact lenses have involved the use of hand or lathe cutting. Such techniques are time consuming and require the use of skilled manpower. Additionally, these techniques frequently fail to provide reproducible results.
It has now been discovered that the disadvantages of the prior art are overcome by the use of a focussed beam of laser-supplied energy to provide the desired peripheral edge on contact lenses. The method of the invention provides a contoured, peripheral edge on the contact lens quickly, reproducibly, and with a minimum of manpower. Preferably, the edge produced by the present invention requires no additional processing (e.g., polishing) to render it physiologically acceptable. As used in the present invention the term "physiologically acceptable edge" means one which is comfortable in the eye, aids in tear pumping under the lens, and has a smooth, continuous profile. Such edges are virtually free from irregularities such as grains, lumps, bumps, grooves, scallops, burrs, pits, bubbles, and so forth when viewed under 50.times.to100.times.magnification.
The use of lasers to machine workpieces is known. See, for example, U.S. Pat. Nos. 3,538,298; 3,571,555; 3,700,850; 3,749,878; 4,307,046, and 4,170,726. However, none of these patents is directed to the formation of peripheral edges on contact lenses.
For example, U.S. Pat. No. 3,538,298 discloses a method for balancing a rotational object by cutting it with a laser pulse having a focal diameter equal to the spacing between two annular grooves in the object. The grooves are employed to eliminate the formation of a recast layer during cutting.
U.S. Pat. No. 3,571,555 discloses an apparatus and technique for laser machining wherein the laser is directed through a dielectric material toward a workpiece. The dielectric material acts as a waveguide to minimize divergence of the laser beam.
U.S. Pat. No. 3,700,850 discloses a method for detecting the amount of material removed from a workpiece by a laser in which sensors attached to the workpiece measure the time it takes for the accoustical shock generated by the laser to traverse the workpiece.
U.S. Pat. No. 3,749,878 discloses a gas assisted laser cutting apparatus wherein a stream of gas and a laser beam are directed through a nozzle toward a workpiece. The gas is used primarily to sweep the debris created by laser cutting away from the machined area.
U.S. Pat. No. 4,307,046 discloses a method of cutting spherical, aspheric and toric optical surfaces on contact lenses so as to simultaneously machine and polish the optical surfaces. The edges of the contact lens are provided by the mold in which the contact lens blank is cast. See FIG. 4.
U.S. Pat. No. 4,170,726 discloses a method for working the outer periphery of a workpiece wherein a workpiece is rotated while being irradiated tangentially with a laser beam. The laser beam melts the workpiece in the area of irradiation. The workpieces illustrated in this patent are cylindrical bodies rather than thin disc-like bodies such as contact lenses.
None of the above patents either recognizes the difficulties encountered in laser cutting polymeric materials or suggests a precise method of overcoming such difficulties. Thus, it has been found that the energy supplied to the polymer by the laser must be controlled so that undesirable heating of the polymer does not occur. While the exact mechanism by which the polymer is cut by the laser is not fully understood, it is believed that polymers generally have low thermal diffusivity and conductivity constants. As a result, heat may build up in the polymer in the area around the point where the laser beam strikes the polymer. If this area (sometimes referred to as the heat-affected zone) extends too deeply beneath the surface of the polymer, the heat build-up will bring this subsurface polymer to a temperature sufficient to cause subsurface explosions thereby forming an unacceptable edge. Accordingly, the energy supplied to the polymer must be carefully controlled so as to minimize the depth of the heat-affected zone.